The personal computer and other digital devices are rapidly becoming key communication tools for millions of users worldwide. The importance of digital and data network communications has greatly increased with the explosion of the Internet. While electronic mail is still a dominant method of interactive computer communications, electronic conferencing and IP-based telephony are becoming increasingly attractive. The adoption of packet switching and its merging with circuit switching helps drive this communications migration. There are many reasons for this, among them pricing advantages due to improved source utilization, seamless transmissions between monomedia and multimedia communications, as well as between human-to-computer (e.g. web-based) and interpersonal interactions.
For interactive multimedia communications on packet-based networks including IP-based telephony, the relevant standard of the Telecommunication Sector of International Organization for Standardization (ITU-T) is the H.323 series of recommendations comprising besides H.323 itself H.225.0 (core message definitions), H.245 (media channel control), H.235 (security framework), H.450.x (supplementary services) and H.332 (extensions for large group conferences).
Multimedia in the circuit switched (CS) domain is realized by the H.324 (incl. H.324/I) recommendation for PSTN- and ISDN-based video conferencing terminals or devices. The H.324 terminal sets up a transparent CS data call, and the user plane connection which is called bearer in the context of third generation (3G) mobile communications, is partitioned by the H.324 terminal application into various media components. The composition of the media components is the result of an in-band negotiation, comprising those components which were offered by the calling user or terminal and supported or accepted by the called user or terminal. An in-band signaling is a signaling where the control signals are sent within a specific logical channel of the user plane connection or bearer. The partitioning of the bearer into logical channels is then performed by a H.223 multiplex protocol, and the control (i.e. setting up, releasing the logical channels etc.) is handled by a H.245 control protocol. The setup of the H.324 multimedia session is divided into two phases. Initially, an outband phase is handled by the “basic telephony” call control (CC) by which the circuit-switched call is set up and a bearer is provided. Then, an inband phase follows, where the peer H.324-terminals negotiate and set up the various media components over the established user plane connection or bearer.
However, such a split of the setup into an outband phase and an inband phase causes problems in the interworking between the packet-switched (PS) domain of e.g. an IM (IP-based Multimedia) subsystem, such as 3GPP (Third Generation Partnership Project) PS domain, and the CS domain. In the IM domain, each media component is carried by a dedicated bearer, which means that the core network “knows” the media components, and the setup of these media components can be handled by the CC outband signaling, whereas in the CS domain the setup of the media components is negotiated by the inband signaling.
A similar problem exists in a pure CS environment, since there is a complex relation between inband and outband negotiation means, which results in inflexible charging of CS multimedia calls. Also, fallback to speech, if the called party does not support multimedia, is complicated.